FileOverlay 1.0.0-beta.1

.NET 10.0 This package targets .NET 10.0. The package is compatible with this framework or higher.
This is a prerelease version of FileOverlay. 
dotnet add package FileOverlay --version 1.0.0-beta.1
                    


                    

                
NuGet\Install-Package FileOverlay -Version 1.0.0-beta.1
This command is intended to be used within the Package Manager Console in Visual Studio, as it uses the NuGet module's version of Install-Package. 
<PackageReference Include="FileOverlay" Version="1.0.0-beta.1" />
For projects that support PackageReference, copy this XML node into the project file to reference the package. 
<PackageVersion Include="FileOverlay" Version="1.0.0-beta.1" />
                    


                            Directory.Packages.props
                        

                    

                
<PackageReference Include="FileOverlay" />
                    


                            Project file
For projects that support Central Package Management (CPM), copy this XML node into the solution Directory.Packages.props file to version the package. 
paket add FileOverlay --version 1.0.0-beta.1
                    


                    

                
#r "nuget: FileOverlay, 1.0.0-beta.1"
#r directive can be used in F# Interactive and Polyglot Notebooks. Copy this into the interactive tool or source code of the script to reference the package. 
#:package FileOverlay@1.0.0-beta.1
#:package directive can be used in C# file-based apps starting in .NET 10 preview 4. Copy this into a .cs file before any lines of code to reference the package. 
#addin nuget:?package=FileOverlay&version=1.0.0-beta.1&prerelease
                    


                            Install as a Cake Addin
                        

                    

                
#tool nuget:?package=FileOverlay&version=1.0.0-beta.1&prerelease
                    


                            Install as a Cake Tool

FileOverlay

A file provider wrapper for ASP.NET Core that enables runtime modification of static files. Useful for rewriting HTML base href attributes when hosting under a path base, while preserving proper HTTP caching headers.

When hosting ASP.NET Core applications (especially SPAs) under a path base like /myapp, you often need to dynamically rewrite the <base href="/"> tag in your HTML to match the deployment path. Traditional solutions either:

  • Require build-time configuration (different builds for different deployments)
  • Use middleware that breaks HTTP caching (loses ETags, LastModified headers)
  • Manually copy and modify files on application startup

FileOverlay solves this by creating a transparent file provider overlay that:

  • ✅ Modifies files at runtime
  • ✅ Preserves all HTTP caching headers (ETags, LastModified)
  • ✅ Works seamlessly with ASP.NET Core's static file middleware
  • ✅ Supports any IFileProvider source

Installation

dotnet add package FileOverlay

Usage Example: Replacing <base href="/"> with PathBase

Here's a complete example showing how to rewrite the base href in your HTML files to use the application's PathBase:

using FileOverlay;

var builder = WebApplication.CreateBuilder(args);

var app = builder.Build();

// Get the PathBase from configuration or environment
var pathBase = app.Configuration["PathBase"] ?? "/";

// Create an overlay that rewrites base href in index.html
var fileProvider = app.Environment.WebRootFileProvider.WithBaseHrefRewrite(
    pathBase: pathBase,
    "index.html" // Can specify multiple files: "index.html", "about.html", etc.
);

// Use the overlay file provider for static files
app.UseStaticFiles(new StaticFileOptions 
{ 
    FileProvider = fileProvider 
});

app.Run();

Complete SPA Example with Fallback Routing

For a typical SPA setup with client-side routing, you'll want to use the full middleware pipeline with MapFallbackToFile:

using FileOverlay;

var builder = WebApplication.CreateBuilder(args);

var app = builder.Build();

// Get the PathBase from configuration or environment
var pathBase = app.Configuration["PathBase"] ?? "/";

// Create an overlay that rewrites base href in index.html
var fileProvider = app.Environment.WebRootFileProvider.WithBaseHrefRewrite(
    pathBase: pathBase,
    "index.html"
);

// Optional (relevant if there are api endpoints that are registered)
app.UsePathBase(pathBase)
   .UseRouting() // Must be called explicitly for PathBase to have effect, see https://andrewlock.net/using-pathbase-with-dotnet-6-webapplicationbuilder/#option-1-controlling-the-location-of-userouting-

app.UseDefaultFiles()
   .UseStaticFiles(new StaticFileOptions 
   { 
       FileProvider = fileProvider 
   });

app.MapFallbackToFile("index.html", new StaticFileOptions 
{ 
    FileProvider = fileProvider 
});

app.Run();

This ensures that:

  • Static files are served with the overlayed file provider
  • Client-side routes (like /products, /about) fall back to the transformed index.html
  • The base href is correctly rewritten in all scenarios

What it does

If your index.html contains:

<!DOCTYPE html>
<html>
<head>
    <base href="/" />
    <title>My App</title>
</head>
<body>
    
</body>
</html>

And you set PathBase to /myapp, the served HTML will automatically become:

<!DOCTYPE html>
<html>
<head>
    <base href="/myapp/" />
    <title>My App</title>
</head>
<body>
    
</body>
</html>

Advanced Usage: Custom Transformations

For more complex scenarios, you can use the low-level API to create custom file transformations:

var overlay = new OverlayFileProvider(app.Environment.WebRootFileProvider);

// Create an overlay for a specific file
var indexFile = overlay.CreateOverlay("index.html");

// Apply custom transformations
indexFile.TransformContent(content =>
{
    // Replace any placeholder with runtime values
    content = content.Replace("{{API_URL}}", app.Configuration["ApiUrl"]);
    content = content.Replace("{{VERSION}}", app.Configuration["Version"]);
    return content;
});

// Use the overlay for serving static files
app.UseStaticFiles(new StaticFileOptions { FileProvider = overlay });

How It Works

  1. Overlay Creation: When you call CreateOverlay(), FileOverlay creates a physical copy of the file in a temporary directory
  2. Transformation: You can then transform the content using TransformContent()
  3. Transparent Serving: The overlay provider intercepts requests for overlayed files and serves the modified versions
  4. Cache Preservation: The overlay preserves the original file's LastModified timestamp, ensuring proper HTTP caching with ETags

Files that are not overlayed are served directly from the original provider without any overhead.

Benefits

  • Runtime Configuration: No need for different builds per environment
  • Proper HTTP Caching: Maintains ETags and LastModified headers for optimal performance
  • SPA Friendly: Perfect for SPAs deployed under path bases
  • Flexible: Works with any IFileProvider implementation
  • Minimal Overhead: Only specified files are copied and modified

Common Scenarios

Hosting Multiple SPAs Under Different Paths

// App 1 at /app1
app.Map("/app1", app1 =>
{
    var fileProvider = env.WebRootFileProvider.WithBaseHrefRewrite("/app1", "index.html");
    app1.UseStaticFiles(new StaticFileOptions { FileProvider = fileProvider });
});

// App 2 at /app2
app.Map("/app2", app2 =>
{
    var fileProvider = env.WebRootFileProvider.WithBaseHrefRewrite("/app2", "index.html");
    app2.UseStaticFiles(new StaticFileOptions { FileProvider = fileProvider });
});

License

MIT

Contributing

Contributions are welcome! Please open an issue or submit a pull request on GitHub.

Product Compatible and additional computed target framework versions.
.NET net10.0 is compatible.  net10.0-android was computed.  net10.0-browser was computed.  net10.0-ios was computed.  net10.0-maccatalyst was computed.  net10.0-macos was computed.  net10.0-tvos was computed.  net10.0-windows was computed. 
Compatible target framework(s)
Included target framework(s) (in package)
Learn more about Target Frameworks and .NET Standard.

NuGet packages

This package is not used by any NuGet packages.

GitHub repositories

This package is not used by any popular GitHub repositories.

Version Downloads Last Updated
1.0.0-beta.1 47 1/29/2026
1.0.0-beta.0 48 1/29/2026